Analysis of simulated partial discharges in a high voltage transformer by Coherence Function

The work presented in this paper deals with the analysis of neutral current for any difference between signals with Partial Discharge (PD) and without Partial Discharge, their frequency response and Coherence Function (CF). The equivalent circuit of a 400 kV class system transformer is determined based on self and mutual inductances, series and shunt capacitances of the winding. The inductive and capacitive parameters related to above circuit are entered into EMTP Program and neutral current is calculated. PD is simulated by adding a small quantity of capacitance between any two nodes comprising a section. The duration of PD is controlled by varying a time controlled opening and closing switch in series with small capacitance. The Fast Fourier Transform and CF are calculated for neutral current with and without PD. The variation in CF confirms the presence of PD.